The present invention relates generally to a method and apparatus for calibrating a streak camera and more particularly to a method and apparatus for calibrating the time axis and intensity linearity of a streak camera over any one of a number of different time scales.
As is known, a streak camera measures light from an event as a function of time on the picosecond scale. The advent of picosecond light pulses, such as modelocked laser pulses and synchrotron radiation, created demand for reliable and compact high-speed streak cameras for temporally and spatially resolved studies of beam diagnostics, high-speed phenomena ensuing from picosecond excitation, and laser-induced plasmas.
The picosecond streak camera is indispensable in many areas of time-resolved studies in scientific research and engineering applications. Researchers in biology, chemistry and physics use the streak camera to measure the fluorescence and absorption kinetics of ultrafast light phenomena.
It is invaluable in the diagnostics of mode-locked and Q-switched laser pulses, pulse progagation studies and laser-plasma interactions, and can be used in the study of implosions in laser fusion experiments. The streak camera may facilitate laser pulse millimeter ranging of complicated and normally inaccessible structures. And, it is useful in diagnostics of high-energy particle beam interactions with matter and in nuclear explosion monitoring.
A streak camera relies on conversion of time information into spatial information. Photons striking the photocathode of the streak tube produce emission of electrons in proportion to the incident light intensity. The electrons are then accelerated into the streak tube via an accelerating mesh and are electrostatically swept at a known rate over a known distance, converting temporal information into spatial information.
These electrons then strike a microchannel plate capable of producing electron multiplication through secondary emission. The secondary electrons released at different times (in relation to the incident electrons) impinge upon a phosphor screen forming the streak image. The streaked luminous output formed on the phosphor screen is viewed by a film back or by an electronic video readout system, which interprets the information as time as a function of position. By viewing the entire luminous event, a streak camera "fingerprints" the time-resolved spectroscopic characteristics of a molecular system.
A discussion of streak camera systems may be found in an article by N. H. Schiller, Y. Tsuchiya, E. Inuzuka, Y. Suzuki, K. Kinoshita, K. Kamiya, H. Lida and R. R. Alfano appearing in the June, 1980, issue of Optical Spectra and an example of a streak camera system may be found in U.S. Pat. No. 4,232,333 to T. Hiruma et al. Other known prior art to the present invention includes U.S. Pat. No. 3,827,075, to O. M. Baycura; U.S. Pat. No. 3,892,468, to M. A. Duguay; U.S. Pat. No. 3,925,727, to M. A. Duguay; and U.S. Pat. No. 4,037,922, to S. A. Claypoole.
In the past, the time axis and intensity linearity of a streak camera has been calibrated by passing a single laser pulse (such as a 6 ps. 530 nm. pulse) through a pair of mirrors of transmission coefficient T (for each mirror) and separated by an air spacing of d. The calibrating pulses produced in this manner make up a train of pulses separated in a time .DELTA..gamma.=2d/c, where c is the velocity of light. The intensity profile of the train is a decaying exponential with each subsequent peak reduced by (1-.UPSILON.).sup.2
For each round trip of the pulse between mirrors, a light pulse of intensity I.sub.k =I.sub.o (1-.UPSILON.).sup.2k is produced, for k=0,1,2 . . . n. Since I.sub.k /I.sub.k+1 =1/(1-.UPSILON.).sup.2 =constant, the envelope formed by the peaks of the pulses follows a single exponential decay as ##EQU1## where the time between peaks t=k.DELTA..tau.. The peaks are used to calibrate the time axis and correct for the intensity variations. For a number of reasons, this technique has not proven to be entirely satisfactory or adequate.
Accordingly, it is an object of this invention to provide a new and improved technique for calibrating the time axis and intensity linearity of a streak camera.